This invention relates generally to compensating electrical circuits for temperature variation and, more particularly, to a system and method of digitally programming currents and voltages that follow a predefined finite temperature coefficient.
Most electrical components use parts having transistor or diodes junctions which have a current/voltage relationship that varies with respect to temperature. Changes in either voltage or current increase the uncertainties at electrical interfaces, and may degrade the performance of circuits. Even though the principles of these temperature dependent current/voltage relationship are well understood, solutions are not always simple due to lot variations in electrical parts, or a limited amount of resources available for temperature compensation. Other parts used in electrical circuits, besides transistors, are subject to performance variations in response to temperature. Examples of components requiring temperature compensation include optoelectronic components (e.g., laser diodes), physical sensors (e.g., pressure sensors), to name but a few.
As current and voltage references comprise a basic and fundamental building block of electronic systems, there consequentially exists a significant amount of prior art. Most prior art circuits describe references that strive to provide currents and voltages that are independent of temperature. However, a small subset of these previous inventions also includes disclosures of references with variable temperature characteristics.
Compensating for temperature characteristics allows the stable operation of electronic components over variations in temperature. Temperature compensation becomes even more critical in circuits requiring a high level of integration, or rapid, low cost, highly reproducible implementation. One of the problems associated with building circuits which compensate for temperature variations, is that temperature compensation circuits themselves are subject to temperature related performance changes.
Digital circuitry has been used in some temperature compensation circuits to provide a constant, or predetermined output, so as to make temperature compensation circuits more stable. Digital programmability permits the temperature characteristics of the reference to be controlled by a microprocessor. Such a capability permits sophisticated control algorithms to be implemented in the microprocessor's software, and permits a microprocressor to configure the reference with specific temperature compensation coefficients previously stored in memory.
Many conventional temperature compensation circuits depend on the adjustment of on-chip resistors to achieve the proper variation in the temperature coefficient of a current. These circuits are often geared more for circuit biasing rather than for being a reference. Circuits providing a predetermined temperature coefficient (tempco), absolute accuracy, and computer controllability have been largely unavailable.
It would be advantageous if a temperature compensation circuit could exactly provide a predetermined finite current or voltage temperature coefficient that is relatively constant with respect to temperature.
It would be advantageous if temperature compensation circuitry could be built using digital hardware to exactly provide predetermined outputs. It would also be advantageous if the digital temperature compensation could provide an exact current or voltage coefficient into response to digital control signals.
Accordingly, a temperature compensation circuit is provided comprising a first digital-to-analog (DAC) circuit to source/sink a compensated temperature dependent current which has been proportionally modified from the first temperature dependent current.
A second DAC source/sinks a compensated temperature independent current which has been proportionally modified from a first temperature independent current. In one aspect of the invention, current is sourced by the second port, in a second aspect the current is sunk.
The two compensated currents are summed to provide a reference current which precisely varies to a predetermined temperature coefficient. When a resistive element, having an impedance matching the tempco impedance of the current source, is added to the circuit, a reference voltage having a predetermined temperature coefficient is provided by shunting the reference current across the resistive element.
The first DAC has an input to accept a first digital control signal. The first DAC modifies the flow of the compensated temperature dependent current in response to the first digital control signals. Likewise, second DAC accepts a second digital control signal. The second DAC varies the flow of the compensated temperature independent current in response to the second digital control signal. Typically, the first and second DACs are responsive to n-bits of selectable control.
In the first aspect of the invention, p (the digital control value) varies from 0 to 2.sup.n -1, and the first and second control lines provide the value p. Then, the compensated currents are proportional to the value of p/N, where N=2.sup.n -1. In this first aspect, the first DAC sources the compensated temperature dependent current, and the second DAC sinks the compensated temperature independent current. The reference current is then equal to the first temperature independent current+second temperature dependent current-second temperature independent current.
In the second aspect of the invention, the first control line provides the value p, in which the second control lines provides the value (N-p). Then, the compensated dependent current in proportional to the value of p/N, and the compensated independent current is proportional to the value of (N-p)/N. In this second aspect, the first DAC sources the compensated temperature dependent current, and the second DAC sources the compensated temperature independent current. The reference current is equal to the compensated temperature independent current+compensated temperature dependent current.
A method of temperature compensation is also provided comprising the steps of:
a) generating a first temperature dependent current; PA1 b) generating a first temperature independent current; PA1 c) generating a compensated temperature dependent current selectively proportional to the first temperature dependent current; PA1 d) generating a compensated temperature independent current selectively proportional to the first temperature independent current; and PA1 e) in response to Steps c) and d), generating a reference current output which precisely varies to a predetermined temperature coefficient. PA1 Typically, other steps precede Step c), of: PA1 b1) introducing a first control signal; and PA1 b2) introducing a second control signal.
Then, Step c) varies the compensated temperature dependent current in response to the first control signal, and Step d) varies the first temperature independent current in response to the second control signal.
Steps b1) and b2) include providing an n-bit first and second control signal, and Steps c) and d) vary the compensated currents over a range of 2.sup.n levels. In the first aspect of the invention, the control signals provide a control bit p, the value of which varies from 0 to 2.sup.n -1. Then, Steps c) and d) include varying the compensated currents in proportion to the value of p/N. The reference current equals the first temperature independent current of Step b)+compensated temperature dependent current of Step c)-compensated temperature independent current of Step d).
In the second aspect of the invention, Step b1) provides the value p, and Step b2) provides the value (N-p). Then, Step c) includes varying the compensated dependent current in proportion to the value of p/N, and Step d) includes varying the compensated independent current in proportion to the value of (N-p)/N. The reference current is equal to the compensated temperature dependent current of Step c)+compensated temperature independent current of Step d).